Integrated circuits are typically formed on a semiconductor substrate such as a silicon wafer or other semiconducting material. In general, layers of various materials which are either semiconducting, conducting or insulating are utilized to form the integrated circuits. By way of example, the various materials are doped, ion implanted, deposited, etched, grown, etc. using various processes. A continuing goal in semiconductor processing is to continue to strive to reduce the size of individual electronic components thereby enabling smaller and denser integrated circuitry.
One technique for patterning and processing semiconductor substrates is photolithography. Such includes deposition of a patternable masking layer commonly known as photoresist. Such materials can be processed to modify their solubility in certain solvents, and are thereby readily usable to form patterns on a substrate. For example, portions of a photoresist layer can be exposed to actinic energy through openings in a radiation-patterning tool, such as a mask or reticle, to change the solvent solubility of the exposed regions versus the unexposed regions compared to the solubility in the as-deposited state. Thereafter, the exposed or unexposed regions can be removed, depending on the type of photoresist, thereby leaving a masking pattern of the photoresist on the substrate. Adjacent areas of the underlying substrate next to the masked portions can be processed, for example by etching or ion implanting, to effect the desired processing of the substrate adjacent the masking material. In certain instances, multiple different layers of photoresist and/or a combination of photoresists with non-radiation sensitive masking materials are utilized.
The continual reduction in feature sizes places ever greater demands on the techniques used to form the features. For example, photolithography is commonly used to form patterned features, such as conductive lines. A concept commonly referred to as “pitch” can be used to describe the sizes of the features in conjunction with spaces immediately adjacent thereto. Pitch may be defined as the distance between an identical point in two neighboring features of a repeating pattern in a straight line cross section, thereby including the maximum width of the feature and the space to the next immediately adjacent feature. However, due to factors such as optics and light or radiation wave length, photolithography techniques tend to have a minimum pitch below which a particular photolithographic technique cannot reliably form features. Thus, minimum pitch of a photolithographic technique is an obstacle to continued feature size reduction using photolithography.
Pitch multiplication, such as pitch doubling, is one proposed method for extending the capabilities of photolithographic techniques beyond their minimum pitch. Such typically forms features narrower than minimum photolithography resolution by depositing spacer-forming layers to have a lateral thickness which is less than that of the minimum capable photolithographic feature size. The spacer-forming layers are commonly anisotropically etched to form sub-lithographic features, and then the features which were formed at the minimum photolithographic feature size are etched from the substrate. Using such technique where pitch is actually halved, such reduction in pitch is conventionally referred to as pitch “doubling”. More generally, “pitch multiplication” encompasses increase in pitch of two or more times and also of fractional values other than integers. Thus, conventionally, “multiplication” of pitch by a certain factor actually involves reducing the pitch by that factor.
Transistor gates are one general type of integrated circuit device component that may be used in many different types of integrated circuitry, for example in memory circuitry such as flash. A flash memory is a type of EEPROM (electrically-erasable programmable read-only memory) that may be erased and reprogrammed in blocks. Many modern personal computers have BIOS stored on a flash memory chip. Such BIOS is sometimes called flash BIOS. Flash memory is also popular in wireless electronic devices as it enables manufacturers to support new communication protocols as they become standardized, and provides the ability to remotely upgrade the devices for enhanced features.
A typical flash memory comprises a memory array that includes a large number of memory cells arranged in row and column fashion. The cells are usually grouped into blocks. Each of the cells within a block may be electrically programmed by charging a floating gate. The charge may be removed from the floating gate by a block erase operation. Data is stored in a cell as charge in the floating gate.
NAND is a basic architecture of flash memory. A NAND cell unit comprises at least one select gate coupled in series to a serial combination of memory cells (with the serial combination being commonly referred to as a NAND string).
Flash memory incorporates charge storage structures into transistor gates, and incorporates control gate structures over the charge storage structures. The charge storage structures may be immediately over gate dielectric. The charge storage structures comprise material capable of storing/trapping charge and which is collectively referred to herein as floating gate material. The amount of charge stored in the charge storage structures determines a programming state. In contrast, standard field effect transistors (FETs) do not utilize charge storage structures as part of the transistors, but instead have a conductive gate directly over gate dielectric material.